PURPOSE: To manufacture a diamond coated tool at low cost by forming a diamond film by using a silicon nitride substrate having the needle crystal structure that the surface becomes a silicon carbide by the vapor phase developing method utilizing a low temp. plasma.

CONSTITUTION: A low temp. plasma is generated by a microwave on the substrate 5 of a silicon nitride 4 having a needle crystal structure whose surface becomes a silicon carbide 3, for instance, an ethanol is used for a raw material gas and the composite time is taken for ten hours in total of three hours with 4SCCM of the flow, 36SCCM hydrogen gas flow, 200W microwave output and 30Torr gas pressure, and 7 hours with 300W microwave output and 40Torr gas pressure. In this case the detection of Si is not found nor of the needle structure of the silicon nitride, and a diamond coating is found. For instance no crack was generated even in case of 3000m cutting with an aluminum alloy being machined at 60m/min cutting speed and 0.5mm depth of cut max.